Final answer:
The statement is true; subduction balances sea-floor spreading by recycling old crust into the Earth's mantle, maintaining the Earth's size constant.
Step-by-step explanation:
The statement is true: as the sea-floor spreads, subduction does indeed prevent the Earth from enlarging. The Earth's crust is made up of tectonic plates that are constantly moving. These plates can either move apart, creating new crust, such as at the mid-Atlantic Rift, or come together, resulting in one plate being forced underneath another into the Earth's mantle—a process known as subduction. So, while the sea-floor is spreading in some places, subduction in other places balances the overall process, maintaining the Earth's size.
Subduction occurs at convergent plate boundaries where one plate with oceanic crust descends beneath another. This old oceanic plate is then re-melted and recycled deep within the Earth's mantle. Examples of such subduction zones include the Japan trench along the coast of Asia. This recycling process, including mantle convection, is vital in ensuring that the material added from the mid-ocean ridges is balanced by the material that is subducted and destroyed, keeping the Earth's size constant.